The present invention relates to a process for purifying a sucrose fatty acid ester (sugar ester). More particularly, the present invention relates to a treatment of a reaction mixture containing a sucrose fatty acid ester to remove the unreacted sucrose, a salt by-produced by neutralization of a catalyst used in the production of the sucrose fatty acid ester, and a volatile component on an industrial scale, and relates to a recovery of the purified sucrose fatty acid ester in the form of a powder.
Sucrose fatty acid esters (sugar esters) useful as surface active agents are prepared industrially at present by either a solvent process wherein sucrose is reacted with a methyl ester of a higher fatty acid having 8 to 22 carbon atoms in the presence of a suitable catalyst in an organic solvent such as dimethylformamide or dimethylsulfoxide, as disclosed in Japanese Patent Publication Kokoku No. 35-13102; or an aqueous medium process wherein sucrose is formed into a molten mixture with a fatty acid salt (soap) using water without using an organic solvent, and is then reacted with a higher fatty acid methyl ester in the presence of a catalyst, as disclosed in Japanese Patent Publication Kokoku No. 51-14485. However, even according to any of these processes, the obtained reaction mixture contains impurities such as the unreacted sucrose, the unreacted fatty acid methyl ester, residual catalyst, soap, free fatty acid, volatile material, etc. in addition to the desired sucrose fatty acid ester. These impurities, at least impurities whose contents exceed the specified amounts must be removed prior to being put on the market. In particular, removal of the unreacted sucrose is very important because of inclusion in a large amount.
The removal of the unreacted sucrose from the reaction mixture containing sucrose fatty acid ester has been generally conducted by utilizing the property that sucrose is slightly soluble in common organic solvents, namely by adding a solvent to the reaction mixture to precipitate the unreacted sucrose and removing the precipitate. This process may be useful for the production on a small scale, but is not suitable for the production on an industrial scale because of problems in handling the solvent, such as labor and time for recovering the solvent, risk of a fire, and environmental sanitation for workers. However, since there is no other useful means, organic solvents have been still used for removal of the unreacted sucrose and other impurities. For example, in Japanese Patent Pulication Kokoku No. 42-11568 and No. 48-10448, it is disclosed that organic solvents such as butyl alcohol, toluene, methyl ethyl ketone and ethyl acetate are effective for purification including removal of the unreacted sucrose. The use of organic solvents is accompanied, for example, by the following disadvantages: (1) risk of explosion and fire, (2) provision of explosion and fire prevention means to electric devices, (3) application of closed system to production equipment for explosion and fire prevention, (4) requirement of fireproof construction for entire building by way of precaution against explosion and fire, (5) rise in fixed cost due to the items (2), (3) and (4), (6) rise in materials cost due to loss of solvent, (7) contamination of the product with remaining solvent, and (8) adverse influence on health of workers, and increase of process steps and cost resulting therefrom.
In view of these circumstances, it has been desired to develop a technique capable of removing the unreacted sucrose and other impurities from the crude reaction mixture without using organic solvents.
Accordingly, it is a primary object of the present invention to establish a technique for simultaneously removing not only the unreacted sucrose, but also the remaining catalyst, salts which is the by-product, of a, volatile components (namely remaining solvents used in the reaction for the production of sucrose fatty acid ester) and other impurities from the crude reaction mixture without using organic solvents.
A further object of the present invention is to provide a process for recovering a sucrose fatty acid ester without using organic solvents from a reaction mixture obtained by the reaction of sucrose and a fatty acid ester according to the organic solvent process or the aqueous medium process.
Another object of the invention is to provide a process for recovering a purified sucrose fatty acid ester in the form of a powder without using orgnaic solvents in purification of the ester.
These and other objects of the present invention will become apparent from the description hereinafter.